Field of the Invention
The present invention relates to an image forming apparatus configured to form images on both sides of a sheet.
Description of the Related Art
In recent years, in image forming apparatus particularly for production use, in a job for forming images on both sides of a sheet, alignment (registration) accuracy of duplex printing for front and back sides is required to be enhanced.
Meanwhile, along with enhancement of accuracy of a conveyance position of the sheet, when edges of the sheets are successively conveyed through the same position of a fixing roller in an axial direction, a surface of the fixing roller may be flawed by cut edges (cut end surfaces) of the sheets. In U.S. Pat. No. 8,204,427, there is described an image forming apparatus, in which, in order to prevent the flaw on the fixing roller, which may be generated by the cut edges of the sheets, the sheet is moved in a direction orthogonal to a conveyance direction on an upstream side of a transfer position, and an image writing start position on a photosensitive member is set in accordance with the shift position.
Further, in Japanese Patent Application Laid-Open No. 2011-180412, there is described an image forming apparatus in which, in the job for forming the images on both the sides of the sheet, shift positions of a first side and a second side are set different from each other, thereby preventing the flaw on the fixing roller.
The shift position of the sheet when the sheet is moved in the direction orthogonal to the conveyance direction of the sheet as described in U.S. Pat. No. 8,204,427 or Japanese Patent Application Laid-Open No. 2011-180412 is determined based on a step angle, a speed reduction ratio, or the number of input steps of a positioning motor of a shift mechanism or other factors. The movement amount of the sheet is computed based on the number of input steps, which are required for moving the sheet to the shift position, and the image writing start position is set in accordance with the movement amount, thereby being capable of performing alignment of an image with respect to the sheet.
However, in actuality, an error occurs for each shift position of the sheet due to, for example, the circularity or the eccentricity of a timing pulley among mechanical components to be used in the above-mentioned shift mechanism. As a result, the positional relationship between the sheet and the image varies for each shift position. When the shift positions of the first side and the second side vary from each other, the alignment accuracy between front and back in the duplex printing may be degraded.